A standard rolling bearing comprises an inner ring fixed to a shaft, and a concentric outer ring, between which a race is formed. Rolling elements are positioned in this race. Conventionally, the outer ring is stationary and the inner ring turns, the opposite assembly being possible. The rolling elements have a very small contact with the inner and outer rings in order to rub the least amount possible and to limit heat buildup.
As described in the application EP 2 749 781the rolling bearing also comprises a lubricant, generally a grease or an oil. The lubrication is an essential element for the correct functioning of the rolling bearing. 70% of the failures of bearings are due to lubrication problems. The purpose of lubrication is to insert a lubricant film (referred to as an oil film) between the rolling elements and the race, in order to limit metal-metal contact, heat buildup, wear and seizing of the elements in contact. The lubricant may also provide protection against oxidation and external pollution.
The rolling bearing for blade roots is one of the various existing bearings used in the aeronautical field. Applications FR 2 862 609 and FR 2 817 233 describe such a bearing.
This rolling bearing is used to orient the blades of a propeller engine, on the basis of the flight phase (takeoff, cruising, landing, etc.). It is positioned in an off-centered manner with respect to the axis of rotation of the engine rotor and turns constantly about the axis of rotation of the rotor. Due to this off-centered position with respect to the axis of rotation of the rotor, the rolling bearing for blade roots is subjected to a large centrifugal force and to high applied stresses. The centrifugal force and the high applied stresses will subject the rolling bearing for blade roots to a high contact pressure, of the order of 3 GPa.
The blade itself is slowly oriented about its longitudinal axis over a low angular range (−30°; +90°) by means of the inner ring and the outer ring of the rolling bearing. Although the relative movement of the two rings is limited, the vibrations make the two rings oscillate continuously with very small displacements generating oscillating conditions.
This high contact pressure and the oscillating conditions result in the lubricant contained in the rolling bearing being driven out of the contact zones. Furthermore, the centrifugal stresses applied to the rolling bearing result in the lubricant being projected outward from the rolling bearing particularly over the high temperature operating range. These projections lead to a loss of lubrication and consequently the deterioration of the rolling bearing for blade roots and the reduction in the service life thereof.
In addition to this high contact pressure and oscillating conditions, the rolling bearing for blade roots is subjected to a large temperature range that may extend from −55° C. (external temperature at altitude) to +170° C. (due to the location close to the engine). The rolling bearing for blade roots must be lubricated over this broad temperature range.
The rolling bearing must therefore be lubricated over a broad temperature range under severe conditions that combine an imposed contact pressure and oscillating conditions. These conditions are antinomic since in order for the rolling bearing to be well lubricated, it is necessary for the lubricant to be able to resupply the contact over the entire temperature operating range, without solidifying at the minimum temperatures nor being expelled at the maximum temperatures under the effect of the centrifugal stresses. On the other hand, in order to withstand the contact pressure and the centrifugal stresses, the lubricant should be as viscous as possible even at the high temperatures encountered during the engine regimes that generate the greatest centrifugation stresses.
Surprisingly, the Applicant has found a complex tribological system that makes it possible to optimize the functioning of a rolling bearing for blade roots. This complex tribological system uses a rolling bearing for blade roots comprising a coating made of a chemical compound of lamellar structure and a lubricant. Specifically, the combination of the coating made of a chemical compound of lamellar structure and of the lubricant enables the rolling bearing for blade roots to be lubricated despite the high contact pressure and the oscillations, for the broad range of temperatures to which it is subjected.
Thus, one object of the invention is, according to a first aspect, a rolling bearing for blade roots extending along a longitudinal direction between a proximal end and a distal end,
the rolling bearing for blade roots comprising:
                an outer ring having an inner surface and an outer surface opposite to the inner surface, the inner surface of the outer ring having an outer race,        an inner ring having an inner surface and an outer surface opposite to the inner surface, the outer surface of the inner ring having an inner race,        a single internal space being defined between the inner surface of the outer ring and the outer surface of the inner ring, the single internal space extending between a proximal end where it is closed off by a proximal sealing system between the outer ring and the inner ring and a distal end where it is closed off by a distal sealing system between the outer ring and the inner ring, said single internal space containing a lubricant,        a row of rolling elements having a contact surface mounted in the single internal space, rolling on the outer and inner races,and characterized in that at least one of the races and/or the contact surface of the rolling elements is (are) coated with a coating made of a chemical compound of lamellar structure.        
The invention also relates to a rolling bearing for blade roots extending along a longitudinal direction between a proximal end and a distal end, the rolling bearing allowing an oscillation of the root about an axis extending along the longitudinal direction relative to a housing,
the rolling bearing for blade roots comprising:
                a single outer ring having an inner surface and an outer surface opposite to the inner surface, the inner surface of the outer ring having a proximal first outer race and a distal second outer race, the first and second outer races being offset relative to one another along the longitudinal direction,        a first inner ring having an inner surface and an outer surface opposite to the inner surface, the outer surface of the first inner ring having a proximal first inner race and a distal seat, the first inner race and the seat being offset relative to one another along the longitudinal direction,        a second inner ring having an inner surface and an outer surface opposite to the inner surface, the outer surface of the second inner ring having a distal second inner race, the second inner ring being fixed to the seat and being maintained axially in abutment against the first inner ring,        a single internal space being defined between the inner surface of the outer ring and the outer surface of the first and second inner rings, the single internal space extending between a proximal end where it is closed off by a proximal sealing system between the outer ring and the first inner ring and a distal end where it is closed off by a distal sealing system between the outer ring and the second inner ring, said single internal space containing a lubricant,        a proximal first row of rolling elements having a contact surface mounted in the single internal space, rolling on the proximal first outer and inner races and a distal second row of rolling elements having a contact surface mounted in the single internal space, rolling on the distal second outer and inner races, the first and second rows of rolling elements being spaced apart from one another along the longitudinal axis,        the inner surface of the first inner ring comprising a shoulder for assembly to a blade root,        the outer surface of the single outer ring comprising a shoulder for assembly to a housing,and characterized in that at least one of the races and/or the contact surface of the rolling elements of one of the rows of rolling elements is (are) coated coated by a coating made of a chemical compound of lamellar structure.        
According to a second aspect, another object of the invention is a lubricating composition for rolling bearing for blade roots characterized in that it comprises a lubricant and a chemical compound of lamellar structure.
According to a third aspect, another object of the invention is a use of a lubricating composition comprising a lubricant and a chemical compound of lamellar structure for lubricating a rolling bearing for blade roots.
According to a fourth aspect, one object of the invention is a process for manufacturing the rolling bearing for blade roots, characterized in that it comprises a step of depositing a coating made of a chemical compound of lamellar structure and a step of applying a lubricant.